helpers: Change patchwork.wsgi to include the apps/ dir in sys.path

Without this, templates will crash as they reference patchwork.<module>
directly.

Also removes the patchwork.wsgi symlink from the root and use the real
file in patchwork.wsgi.conf instead, as suggested by Dirk Wallenstein.

Signed-off-by: Guilherme Salgado <guilherme.salgado@linaro.org>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>